Global Navigation Satellite Systems (GNSS) include the Global Positioning System (GPS), the GLONASS system, the proposed Galileo system, and the proposed Beidou (Compass) system.
The Global Positioning System completed its original design goals when it attained full operational capability in 1995. Technical advances and new demands on the system have since led to a modernization effort. The GPS modernization project involves new ground stations and new satellites, with additional navigation signals and improved accuracy and availability. The first GPS satellite with three-frequency capability including the new L5 frequency, GPS Block IIF-1, is expected to be launched in the summer of 2009. The new civilian-use L5 signal is expected to improve signal structure for enhanced performance, with higher transmission power and wider bandwidth than the L1 and L2C signals to better manage interference than with L2. Launch of additional three-frequency GPS satellites is planned, with a full three-frequency constellation probably available only 5-7 years later. The GPS signals are each modulated on a separate carrier and are thus individually tracked in a receiver. The GPS system is designed to accommodate two levels of service: civilian and military.
The European Galileo satellite system will have similar capabilities, but may not provide them all free-to-air. The Galileo launch schedule is lagging behind the original plan. To date only two Galileo validation element satellites GIOVE-A and GIOVE-B have been launched. An important difference from the GPS system is that the Galileo system's signal structure is planned to have ten different signals transmitted in four frequency bands, to accommodate four levels of service. The Galileo E1 and E2 signals are on separate carriers in the E2-L1-E1 band (sometimes called the L1 band). The Galileo E1 signal has a wider signal frequency span than the GPS L1 C/A signal, so that a simple receiver processing the main lobe of Galileo E1 Open Service (OS) signal will consume more power than its equivalent processing of the main lobe of the GPS C/A signal. The Galileo E6 signal is on a separate carrier in the E6 band carrying Commercial Service (CS) and Public Regulated Service (PRS) signals. In contrast, the Galileo E5a signal and E5b signal are on a single carrier in the L5-E5a-E5b band. The E5a and E5b signals two spectral components produced by a single modulation called alternate binary offset carrier (altBOC) modulation. The altBOC modulation offers a constant envelope while allowing receivers to differentiate the two spectral lobes. The altBOC modulation allows coherent tracking of the whole E5a+E5b signal as a single wideband signal or non-coherent tracking of each of the E5a and E5b signals separately. The E5a and E5b signals are available to the Open Service (OS), Commercial Service (CS) and Safety of Life (SOL) services.
The Galileo system is the only system currently proposed to transmit a signal in a band having a first signal and a second signal which can be tracked as a single wideband signal or can be tracked separately. The Chinese Compass system is in the early stages of testing and its signal structure may be subject to change. The Russian GLONASS system is expected to have additional frequency capabilities at some time in the future and its signal structure may also be subject to change. India is also planning a GNSS system.
Improved methods and apparatus for processing GNSS signals are desired, particularly to improve ambiguity estimation of GNSS signals having a first signal and a second signal in a band which can be tracked as a single wideband signal or can be tracked separately.